JP7536397B2 - Transmitter output power control mechanism - Google Patents

Description

This invention relates to an output power control mechanism for a transmitter, and in particular to an output power control mechanism for a transmitter that uses a digital predistortion method to compensate for distortion in a transmission signal.

For example, in transmitters that transmit signals at particularly high power using shortwave (HF: short for High Frequency), a power amplifier is provided to amplify the signal to be transmitted, and a linear amplifier may be used in the power amplifier. In this case, since the linear amplifier itself does not have a very high linearity, a digital pre-distortion (DPD: short for Digital Pre-Distortion) method is used to suppress the nonlinear distortion that occurs in the linear amplifier, and distortion compensation of the transmission signal is performed (see, for example, Patent Document 1).

JP 2014-155101 A

However, in order to properly perform distortion compensation in the DPD system, the input level of the signal to be transmitted that is input to the DPD must be the same as the input level of the feedback signal used in the DPD process, but there is a problem in that adjusting the input level takes time.

The object of this invention is to provide an output power control mechanism for a transmitter that can adjust the input level of the signal to be transmitted and the input level of the DPD feedback signal in the distortion compensation process of the DPD system in a short period of time, and thus can perform distortion compensation of the DPD system appropriately.

In order to solve the above problem, the invention described in claim 1 is an output power control mechanism for a transmitter, comprising: a DPD processing section that performs distortion compensation processing on an input signal by a digital predistortion method; an upconverter that converts the signal after the distortion compensation processing output from the DPD processing section to a high frequency; an output VGA that adjusts the gain of the signal after the frequency conversion output from the upconverter; a power amplifier that amplifies the signal after the gain adjustment output from the output VGA to an output signal that is supplied to an antenna and transmitted via the antenna; and a feedback VGA that adjusts the gain of a feedback signal that is a signal obtained by folding back a portion of the output signal output from the power amplifier and is used in the distortion compensation processing, and supplies the feedback signal to the DPD processing section, and is characterized in that an initial value used in a convergence calculation to determine the gain value of the feedback VGA to make the input levels of the input signal and the feedback signal input to the DPD processing section the same is calculated using a function that uses the frequency value of the high frequency as a variable.

According to this invention, an initial value that changes depending on the value of the output signal frequency after frequency conversion in the upconverter is used, and a convergence calculation is performed to make the level of the input signal input to the DPD processing unit equal to the level of the feedback signal.

The invention described in claim 2 is characterized in that in the transmitter output power control mechanism described in claim 1, the upconverter frequency-converts the signal after the distortion compensation process output from the DPD processing unit from an audio frequency to a radio frequency.

The invention described in claim 3 is characterized in that in the transmitter output power control mechanism described in claim 1, the high frequency is a short wave.

The invention described in claim 4 is characterized in that in the transmitter output power control mechanism described in claims 1 to 3, the function is a polynomial of second degree or higher.

According to the invention described in claim 1, an initial value that changes depending on the value of the output signal frequency after frequency conversion in the upconverter is used, so that it is possible to perform a convergence calculation in a short time to make the level of the input signal input to the DPD processing unit equal to the level of the feedback signal, and thus it is possible to perform pre-distortion compensation processing in the DPD processing unit appropriately.

According to the invention described in claim 2, the above-mentioned effects can be achieved in a transmitter in which an upconverter converts frequencies from audio frequencies to radio frequencies.

According to the invention described in claim 3, it is possible to achieve the above-mentioned effects in a transmitter that transmits signals in the shortwave frequency band.

According to the invention described in claim 4, it is possible to appropriately set a function for calculating the initial value used in the convergence calculation, and it is possible to perform the convergence calculation in a shorter time by using a more appropriate initial value.

1 is a functional block diagram showing a schematic configuration of a transmitter including a transmitter output power control mechanism according to an embodiment of the present invention;

The present invention will be described below based on the illustrated embodiment.

Figure 1 is a functional block diagram showing the schematic configuration of a transmitter 1 including a transmitter output power control mechanism according to an embodiment of the present invention.

Transmitter 1 is a device that generates and amplifies a radio signal in the shortwave (HF: short for High Frequency; for example, about 1.6 to 30 MHz) frequency band that corresponds to the input signal to be transmitted (the input signal in FIG. 1), and outputs/transmits the signal from antenna 9. It mainly comprises a control unit 2, an A/D converter 3, a DSP unit 4, a first D/A converter 5, a second D/A converter 6, an output VGA 7, a power amplifier 8, and an antenna 9.

The transmitter output power control mechanism according to the embodiment includes a DPD processing unit 45 that performs distortion compensation processing on an input signal using a digital predistortion method, an upconverter 46 that converts the signal after distortion compensation processing output from the DPD processing unit 45 to a high frequency, an output VGA 7 that adjusts the gain of the signal after frequency conversion output from the upconverter 46, a power amplifier 8 that amplifies the signal after gain adjustment output from the output VGA 7 to an output signal that is supplied to an antenna 9 and transmitted via the antenna 9, and a feedback VGA 43 that adjusts the gain of a feedback signal that is a signal obtained by folding back a portion of the output signal output from the power amplifier 8 and is used in distortion compensation processing, and supplies the feedback signal to the DPD processing unit 45. The initial value used in the convergence calculation to find the gain value of the feedback VGA 43 to make the input levels of the input signal and the feedback signal input to the DPD processing unit 45 the same is calculated using a function that uses the high frequency frequency value as a variable.

The control unit 2 is a mechanism for controlling each part of the transmitter 1, and includes a central processing unit 21 (CPU: short for Central Processing Unit), a ROM 22 (ROM: short for Read Only Memory), a RAM 23 (RAM: short for Random Access Memory), and an I/O 24 (I/O: short for Input/Output).

The control unit 2 uses the RAM 23 as a working area as necessary by executing a program for controlling the operation of the transmitter 1 stored in the ROM 22 via the central processing unit 21, and controls the start, content, and end of processing of each part of the transmitter 1 in accordance with the program. Note that, for ease of viewing, the signal lines between the control unit 2 and each part of the transmitter 1 are not shown in FIG. 1, but the control unit 2 and each part of the transmitter 1 are electrically connected to each other via the I/O 24.

The A/D converter 3 (Analog/Digital converter) receives an input of a feedback signal, which is a signal obtained by folding back a part of the signal output from the power amplifier 8 (the output signal in FIG. 1) (in other words, feedback), converts the input feedback signal from an analog signal to a digital signal, and outputs the feedback signal after digital conversion processing. The feedback signal is obtained by folding back a part of the signal output from the power amplifier 8, for example, using a coupler.

The DSP unit 4 (DSP: Digital Signal Processor) is a mechanism that performs pre-distortion compensation using digital signal processing to cancel the non-linear distortion characteristics of the power amplifier 8 in order to correct the non-linear distortion characteristics of the power amplifier 8 and linearize the power amplifier 8, and includes an amplifier 41, a downconverter 42, a feedback VGA 43, a comparison unit 44, a DPD processing unit 45, an upconverter 46, and a level control unit 47.

The amplifier 41 receives an input signal provided as a digital electrical signal, amplifies the input signal, and outputs the amplified input signal.

The down converter 42 (D/C) receives the feedback signal after digital conversion processing output from the A/D converter 3, converts the frequency of the input feedback signal from high frequency/radio frequency (RF: short for Radio Frequency; for example, shortwave frequency band, specifically, about 1.6 to 30 MHz) to low frequency/audio frequency (AF: short for Audio Frequency; for example, about 0.3 to 3 kHz), and outputs the feedback signal after low frequency conversion processing.

In addition, transmitter 1 may have multiple channels, and multiple high frequency/radio frequencies (RF) corresponding to each of the multiple channels may be used.

The feedback VGA 43 (VGA: Variable Gain Amplifier) receives the feedback signal after low-frequency conversion processing output from the downconverter 42, and also receives the input level control signal output from the level control unit 47, adjusts the gain of the feedback signal based on the input level control signal, and outputs the feedback signal after gain adjustment.

The comparator 44 receives the amplified input signal output from the amplifier 41 and the gain-adjusted feedback signal output from the feedback VGA 43, detects the difference in levels between these signals, and outputs a level difference signal. The amplified input signal output from the amplifier 41 and the gain-adjusted feedback signal output from the feedback VGA 43, which are input to the comparator 44, are supplied to the downstream DPD processor 45.

The DPD processing unit 45 is a mechanism that estimates the input/output characteristics of the power amplifier 8 based on the input signal and the feedback signal, and performs pre-distortion compensation using the digital pre-distortion (DPD) method to give the input signal the inverse characteristic of the nonlinear distortion characteristic that occurs in the signal at the power amplifier 8.

The digital predistortion method is a method of performing predistortion compensation in which the distortion compensation coefficient is adaptively updated so as to reduce the difference between a signal (i.e., a feedback signal) that is a folded portion of the signal output from a power amplifier in the transmission system (specifically, power amplifier 8 in the example shown in Figure 1) and the signal to be transmitted (i.e., the input signal), and in particular, is a method in which the predistortion compensation process is performed in the digital domain. Specifically, the distortion compensation coefficient is a coefficient in an inverse model that is estimated as a model that shows the inverse characteristics of the input/output characteristics of power amplifier 8.

The inverse characteristic of the nonlinear distortion characteristic given to the input signal by the DPD processing unit 45 is controlled based on the distortion compensation coefficient set in the DPD processing unit 45, and the nonlinear distortion occurring in the signal in the power amplifier 8 is compensated for by the pre-distortion compensation process by the DPD processing unit 45. The DPD processing unit 45 outputs a signal compensated with the inverse characteristic to the distortion characteristic of the power amplifier 8. Then, by giving the signal compensated with the inverse characteristic to the distortion characteristic of the power amplifier 8 to the power amplifier 8, a signal with suppressed distortion is output from the power amplifier 8.

The digital predistortion method itself is a well-known technique, and while there are various specific methods and procedures for predistortion compensation processing, this invention is not limited to a specific method or procedure, so a detailed explanation of the predistortion compensation processing will be omitted. In this invention, as long as the input signal and feedback signal are used to perform predistortion compensation, the specific configuration of the DPD processing unit 45 and the specific content of the predistortion compensation processing can be any type.

The DPD processing unit 45 receives the amplified input signal output from the amplifier 41 via the comparison unit 44 and the gain-adjusted feedback signal output from the feedback VGA 43, performs pre-distortion compensation processing while estimating the input/output characteristics of the power amplifier 8 based on the input signal and the feedback signal (in other words, while adaptively updating the distortion compensation coefficient), and outputs the signal after distortion compensation processing.

The up converter 46 (U/C) receives the post-distortion compensation signal output from the DPD processing unit 45, converts the frequency of the post-distortion compensation signal from low frequency/audio frequency (AF) to high frequency/radio frequency (RF), and outputs the post-high frequency conversion signal (called the "high frequency distortion compensated signal").

The first D/A converter 5 (Digital/Analog converter) receives the high-frequency distortion-compensated signal output from the upconverter 46, converts the input high-frequency distortion-compensated signal from a digital signal to an analog signal, and outputs the high-frequency distortion-compensated signal after analog conversion processing.

The second D/A converter 6 (Digital/Analog converter) receives the output level control signal output from the level control unit 47, converts the input output level control signal from a digital signal to an analog signal, and outputs the output level control signal after analog conversion processing.

The output VGA 7 receives the high-frequency distortion-compensated signal output from the upconverter 46 and passed through the first D/A converter 5, and also receives the output level control signal output from the level control unit 47 and passed through the second D/A converter 6, adjusts the gain of the high-frequency distortion-compensated signal based on the input output level control signal, and outputs the high-frequency distortion-compensated signal after gain adjustment.

The power amplifier 8 receives the gain-adjusted high-frequency distortion-compensated signal output from the output VGA 7, amplifies the input high-frequency distortion-compensated signal, and outputs it as a signal to be supplied to the antenna 9. The signal output from the power amplifier 8 (the output signal in FIG. 1) is transmitted/radiated via the antenna 9 electrically connected to the output terminal of the power amplifier 8.

The level control section 47 of the DSP section 4 is a mechanism for generating signals that control the feedback VGA 43 and the output VGA 7 so that the levels of the amplified input signal input to the DPD processing section 45 and the feedback signal after gain adjustment are the same, that is, so that the difference in level between the two signals is zero (or falls within a specified range).

The level control unit 47 receives the level difference signal output from the comparison unit 44, i.e., a signal representing the difference between the level of the amplified input signal output from the amplifier 41 and the level of the gain-adjusted feedback signal output from the feedback VGA 43, and generates a combination of an input level control signal that controls the feedback VGA 43 and an output level control signal that controls the output VGA 7 based on the level difference signal so as to make the difference between the levels of the two signals zero (or within a predetermined range).

The combination of the input level control signal and the output level control signal is generated by setting the output level control signal as a command to adjust the gain of the output VGA7, and also by setting the input level control signal as a command to adjust the gain of the feedback VGA43 so that the difference between the level of the input signal output from the amplifier 41 and input to the DPD processing unit 45 under the gain condition of the output VGA7 set above and the level of the feedback signal output from the feedback VGA43 and input to the DPD processing unit 45 becomes zero (or falls within a predetermined range). Note that the gain of the output VGA7 may be set based on a command from the control unit 2 so that the level of the signal output from the output VGA7 varies, specifically, for example, by detecting the output signal in front of the antenna 9 and calculating the output power (i.e., the difference between the forward wave power and the reflected power), and the level of the signal output from the output VGA7 gradually increases so that the calculated output power becomes the set target value.

At this time, the level control unit 47 uses a predetermined initial value to perform a convergence calculation so that the difference between the level of the input signal and the level of the feedback signal becomes zero, and based on the result of the convergence calculation, sets and generates an input level control signal as a command to adjust the gain of the feedback VGA 43. The initial value of the convergence calculation is a value equivalent to the value of the gain of the feedback VGA 43.

Here, according to the inventor's knowledge, the gain value of the feedback VGA 43 obtained as a result of the convergence calculation (in other words, the value for adjusting the gain) differs depending on the value of the high frequency/radio frequency (RF) after the frequency conversion in the upconverter 46. For this reason, the initial value used in the convergence calculation is set to a different value depending on the value of the high frequency/radio frequency (RF) after the frequency conversion in the upconverter 46, so that the convergence calculation can be performed in a short time. The reason why the appropriate value of the gain of the feedback VGA 43 (in other words, the appropriate value for adjusting the gain) differs depending on the value of the high frequency/radio frequency (RF) after the frequency conversion in the upconverter 46 is that, for example, at least some of the elements constituting the circuit related to the feedback signal in the transmitter 1 may cause a level fluctuation that behaves differently depending on the value of the frequency. In other words, since the feedback signal has a frequency characteristic according to the magnitude of the output frequency (for example, a wide range such as 1.6 to 30 MHz), it is not as theoretically calculated, and convergence calculation is required so that the difference between the level of the input signal and the level of the feedback signal becomes zero.

Based on the above knowledge, the level control unit 47 uses a value calculated using a function whose variable is the value of the high frequency/radio frequency (RF) after frequency conversion in the upconverter 46 as the initial value used in the convergence calculation. Then, when the transmitter 1 has multiple channels and uses multiple high frequency/radio frequencies (RF) corresponding to each of the multiple channels, an initial value used in the convergence calculation is calculated for each of the multiple high frequency/radio frequencies (RF).

To calculate the initial value used in the convergence calculation, a function having the high frequency/radio frequency (RF) value after frequency conversion as a variable may be used, specifically, for example, a first-order polynomial such as the following formula 1A, a second-order polynomial such as the following formula 1B, a third-order polynomial such as the following formula 1C, a fourth-order polynomial such as the following formula 1D, or a fifth-order or higher polynomial. In the following formulas 1A to 1D, x is the frequency value [MHz], y is the initial value used in the convergence calculation, _a1 , _a2 , _a3 , and _a4 are coefficients, and c is a constant term.
(Math 1A) y=a ₁ x+c
(Math. 1B) y=a ₂ x ² +a ₁ x+c
(Math. 1C) y=a ₃ x ³ +a ₂ x ² +a ₁ x+c
(Math 1D) y=a ₄ x ⁴ +a ₃ x ³ +a ₂ x ² +a ₁ x+c

In order to determine the values of coefficients _a1 , _a2 , _a3 , ... and the value of the constant term c of a polynomial (called an "initial value calculation formula") as a function for calculating an initial value used in the convergence calculation, the level control unit 47 actually performs a convergence calculation for a predetermined (in other words, specific) frequency value x to obtain a gain value (a value for adjusting the gain) of the feedback VGA 43 obtained as a result of the convergence calculation, and obtains a plurality of combinations of the frequency value x and the gain value y of the feedback VGA 43. Then, the plurality of combinations are used to determine the values of coefficient _{a1, etc. and the value of the constant term c by, for example, regression analysis, as the coefficients and constant terms of the approximation formula expressing the relationship between x and y} .

According to the inventor's knowledge, in order to obtain a good approximation of the relationship between the frequency value x and the gain value y of the feedback VGA 43 obtained as a result of the convergence calculation, it is preferable to use a polynomial of second order or higher, and it is particularly preferable to use a fourth order polynomial.

The initial values used in the convergence calculation may be calculated in advance for each frequency value using an initial value calculation formula, and then may be specified for each frequency value in a program for controlling the operation of the transmitter 1 or in a data file/settings file stored in the ROM 22 of the control unit 2, and referenced when performing the convergence calculation.

Alternatively, the initial value used in the convergence calculation may be calculated by the level control unit 47 using the high frequency/radio frequency (RF) value after frequency conversion, after an initial value calculation formula (including the values of coefficients _a1 , _a2 , _a3 , ... and the value of constant term c) is specified in a program for controlling the operation of the transmitter 1 or stored in the level control unit 47.

As part of the process related to the convergence calculation, first, the level control unit 47 sets the gain of the output VGA 7 (possibly based on a command from the control unit 2) and generates an output level control signal as a command to adjust the gain of the output VGA 7 according to the setting.

Next, the control unit 2 sets the radio frequency/radio frequency (RF) value after frequency conversion in the upconverter 46, identifies an initial value corresponding to the radio frequency/radio frequency (RF) value from among the initial values defined in the program or data file/setting file, or substitutes the radio frequency/radio frequency (RF) value for the variable x in the initial value calculation formula to calculate the value of y.

The control unit 2 then uses the value specified above and the calculated value of y as initial values to perform a convergence calculation to determine the appropriate value for the gain of the feedback VGA 43 such that the difference between the level of the input signal output from the amplifier 41 and input to the DPD processing unit 45 under the gain conditions of the output VGA 7 set above and the level of the feedback signal output from the feedback VGA 43 and input to the DPD processing unit 45 becomes zero.

Specifically, the convergence calculation is performed according to the following outline.
1) Switch the input of amplifier 41 to the nominal level of the internal tone and fix the gain at 0xFFFF.
2) The gain of the high frequency distortion compensated signal of the output VGA 7 is set so that the output power becomes the set power (the output value set here is not changed).
3) The gain of the feedback signal of the feedback VGA 43 is adjusted so that the output values of the amplifier 41 and the feedback VGA 43 match.

Note that although the initial value of the convergence calculation and the appropriate value of the gain of the feedback VGA 43 as a result of the convergence calculation may be affected by (changes in) the gain of the output VGA 7, this is not considered to be a major issue since power calibration is usually set at the rated power of the device.

The control unit 2 generates an input level control signal as a command to adjust the gain of the feedback VGA 43 based on the results of the convergence calculation.

Then, the control unit 2 outputs the input level control signal, which is a combination of the input level control signal that controls the feedback VGA 43 and the output level control signal that controls the output VGA 7, to the feedback VGA 43, and outputs the output level control signal to the output VGA 7 via the second D/A converter 6.

The output VGA 7 performs gain adjustment processing on the high-frequency distortion-compensated signal based on the output level control signal to set the output power, and the feedback VGA 43 performs gain adjustment processing on the feedback signal based on the input level control signal and outputs it, so that the level of the amplified input signal input to the DPD processing unit 45 and the level of the gain-adjusted feedback signal become the same (or the difference in level between the two signals falls within a specified range).

The transmitter output power control mechanism described above uses an initial value that changes depending on the value of the high frequency/radio frequency (RF) after frequency conversion in the upconverter 46, making it possible to perform convergence calculations in a short time to make the level of the input signal input to the DPD processing unit 45 equal to the level of the feedback signal, thereby enabling proper pre-distortion compensation processing in the DPD processing unit 45.

Although the embodiment of the present invention has been described above, the specific configuration is not limited to the above embodiment, and even if there are design changes within the scope of the present invention, they are included in the present invention. For example, the above embodiment is described by taking as an example a case in which the transmitter output power control mechanism of the present invention is applied to a transmitter 1 whose schematic configuration is shown in FIG. 1, but the configuration of a transmitter to which the present invention can be applied is not limited to the example shown in FIG. 1. In other words, the present invention can be applied to any transmitter that has a configuration equivalent to the feedback VGA 43, DPD processing unit 45, upconverter 46, output VGA 7, and power amplifier 8 in the above embodiment.

In addition, in the above embodiment, the transmitter 1 generates and transmits a radio signal in the shortwave frequency band (approximately 1.6 to 30 MHz), and also performs frequency conversion between high frequency/radio frequency (RF; approximately 1.6 to 30 MHz) and low frequency/audio frequency (AF; approximately 0.3 to 3 kHz). However, the frequencies handled by the transmitter 1 are not limited to the frequencies exemplified in the above embodiment.

1 Transmitter 2 Control unit 21 Central processing unit 22 ROM
23 RAM
24 I/O
3 A/D converter 4 DSP section 41 Amplifier 42 Down converter (D/C)
43 Feedback VGA
44 Comparison section 45 DPD processing section 46 Up-converter (U/C)
47 Level control section 5 First D/A converter 6 Second D/A converter 7 Output VGA
8 Power amplifier 9 Antenna

Claims (4)

A DPD processing unit that performs distortion compensation processing on an input signal by a digital predistortion method;
an up-converter that converts the distortion-compensated signal output from the DPD processing unit into a high frequency signal;
an output VGA for adjusting a gain of the frequency-converted signal output from the up-converter;
a power amplifier that amplifies the gain adjusted signal output from the output VGA to an output signal that is provided to an antenna and transmitted via the antenna;
a feedback VGA that adjusts a gain of a feedback signal used in the distortion compensation processing, the feedback signal being a signal obtained by folding back a part of the output signal output from the power amplifier, and supplies the feedback signal to the DPD processing unit;
an initial value used in a convergence calculation for determining a gain value of the feedback VGA for equalizing the input levels of the input signal and the feedback signal input to the DPD processing unit is calculated using a function having a frequency value of the high frequency as a variable;
1. A transmitter output power control mechanism comprising: The up-converter converts the signal after the distortion compensation processing output from the DPD processing unit from an audio frequency to a radio frequency.
2. The transmitter output power control mechanism of claim 1. The high frequency is a short wave.
2. The transmitter output power control mechanism of claim 1. The function is a polynomial of second degree or higher.
A transmitter output power control mechanism as claimed in any one of claims 1 to 3.